The present invention relates to a diffraction grating in which a number of grating grooves are ruled on a spherical surface, and, more particularly, to improvements in a method of manufacturing a concave diffraction grating.
A concave diffraction grating is a dispersive element usually obtained by ruling rectilinear grooves which are equally spaced from and parallel to one another with respect to a plane lying in contact with a grating blank sphere at the center of a sphere. It has a light dispersion property as a diffraction grating and also a light focusing property as a concave mirror, and does not require a collimating and focusing optical element composed of a lens, a concave mirror, etc. necessary for a plane grating spectrometer. For this reason, a concave diffraction grating is indispensable in a spectrometer for the vacuum ultraviolet region in which the reflection factor on a metal surface is generally very low. In addition, it has wide applications such as the simultaneous spectral measurement of multiple wavelengths and a simple spectrometer having a small number of optical components.
However, with a spectrometer employing the prior-art concave diffraction grating in which equally-spaced and parallel rectilinear grooves are ruled with respect to a plane lying in contact with the grating blank sphere at the center thereof, it is inevitable that spectra obtained are usually attended with many aberrations, especially astigmatism. This has been considered to be a significant defect of the concave diffraction grating.
FIG. 1 shows a local curve of diffracted light where a light source is placed at the center of curvature of a prior-art concave diffraction grating. For the prior-art concave diffraction grating 9, the tangential focal curve T (in a direction perpendicular to the grooves) is a circle R, a so-called Rowland circle, which passes through the center of O' of curvature of the sphere and the center O of the diffraction grating and whose diameter is the radius OO' of curvature of the sphere. The sagittal focal curve S (in the direction parallel to the grooves) is on a straight line lying in contact with the Rowland circle R at the center of curvature. Therefore, the diffracted image of light emerging from the center of curvature is not focused on one point, and the spectrum on the Rowland circle being usually used for the spectral analysis is always attended with astigmatism.
In order to eliminate the astigmatism in the concave grating spectrometer, a number of research attempts have hitherto been made. There have been many reports including a method in which parallel rectilinear grooves are ruled on the sphere and the groove intervals are different from each other, a method in which an aspherical surface such as toroidal surface and ellipsoidal surface is employed for the grating blank, and a method in which curved grooves are formed on the sphere. In putting these into practical use, however, numerous problems have been encountered due to restrictions on the range free from aberration, technical difficulties in fabrication, etc.
On the other hand, it has become possible in recent years to fabricate a diffraction grating by the application of laser holography technology. In this method of fabrication, interference fringes by a laser beam are formed on a plane or concave surface, the interference fringes are transformed into unevenness in a photographic emulsion plate or photoresist, and a metal film is thereafter formed on its surface by, for example, vacuum evaporation. Particularly in the case of a concave diffraction grating, it has become possible to manufacture a product which has a light focusing characteristic different from that attained by the conventional mechanical ruling.
FIG. 2 shows a local curve of diffracted light in the case where a light source is placed at the center of curvature of a typical holographic concave diffraction grating.
In the holographic concave diffraction grating 9', two points at which the tangential focal curve T and the sagittal focal curve S coincide are usually present in addition to the center O' of curvature. The diffracted image of light emerging from the center O' of curvature is formed at these points completely stigmatically. Where the light source is located at any of the three points, all the diffracted images obtained at the respective points are stigmatic.
The prior-art holographic concave diffraction grating 9', however, is restricted in the diffraction wavelength and the image forming position at which a stigmatic spectrum is obtained, by the wavelength of the laser used at the manufacture. A further disadvantage is that since the sectional shape of the grating grooves is the unevenness produced from the interference fringes by the photographic treatment, there is not as free a selection of the shape of the groove section as for a mechanical ruling, which makes it impossible to achieve a high diffraction efficiency in an arbitrary wavelength region.